EP0712260A1 - Procedure for insertion of cells in an ATM-stream apparatus for its execution - Google Patents

Abstract

The method involves performing real-time processing on incoming cell trains to form a characteristic message for each received cell which provides cell type information. The message also stores a subsequent processing time. Real time processing is performed on the generated messages to generate a characteristic cell which is transmitted each time a predetermined number of user cells has been transmitted. Characteristic cells are grouped to suppress the number of corresponding empty cells. Otherwise, a waiting empty cell is transmitted or an empty cell is generated.

Description

The subject of the present invention is a method of inserting cells into a cell flow of ATM ("Asynchronous Transfer Mode") type for the purpose, for example, of measuring the quality of a network or of equipment.

It is recalled that the transmission according to the ATM standard is done by transfer of blocks of information of constant length. The elementary quantity of information transmitted is 48 bytes. The block transmitted in ATM is called cell and is identified by a 5 byte label called header. The total length of a cell is therefore 53 bytes.

The ATM standard allows time-division multiplexing at several levels by organizing cell flows in a virtual path VP ("Virtual Path") and virtual channel VC ("Virtual Channel").

Thus all ATM cells can be assigned to a VP virtual path from 4096 and to a VC virtual channel from 65536. The cell and virtual channel numbers of the cell are contained in the 12 and 16 bit fields respectively of the on your mind. Thus, a header of 5 bytes whose bits are numbered from 0 to 7 contains the virtual path number VPI defined on 12 bits, the virtual channel number VCI defined on 16 bits, a cell type indicator PT (" Payload Type ") defined on three bits, a CLP priority indicator (" Cell Loss Priority ") defined on 1 bit as well as a HEC error correction code defined on 8 bits and relating to the previous 4 bytes.

An ATM network allows the transmission of digital information with various speeds which can be very high, which can indeed go up to 155 Mbit / s.

According to the ATM standard, it is planned to insert maintenance flows for the ATM layer. These flows are flows F4 and F5.

Verification of a virtual path is performed by the F4 flow, using OAM cells sent on a reserved virtual channel (VCI = 4 for an end-to-end F4 flow, VCI = 3 for a sub F4 flow -network).

On the other hand, the maintenance flows relating to a given virtual channel (flow F5) take the same path as the useful cells. They are distinguished by a particular coding of the PT field.

The insertion of a flow of characteristic cells, whether maintenance flow or other, can lead to reducing the speed offered to users. In fact, the insertion is made in a forced manner at the expense of the speeds offered to users.

The subject of the present invention is a method of inserting characteristic cells making it possible to carry out the insertion in real time while not reducing the bit rate of the flows offered to users whatever these bit rates.

The invention also relates to a device for implementing the method.

Thus, according to the invention, the insertion method is mainly characterized in that each time a characteristic cell insertion is made, an empty cell of the incident flow is deleted, the deletion not necessarily taking place immediately after the insertion, the number of empty cells to be deleted being counted.

• effectuer un pré-traitement sur le flux entrant pour d'une part, former un message caractérisant chaque cellule reçue et donnant notamment une information sur le type de cellule et d'autre part, sélectionner les cellules usagers et les stocker chacune le temps du traitement qui va suivre,
• effectuer un traitement à partir des messages formés pour générer une cellule caractéristique, l'émettre à chaque fois qu'un nombre déterminé de cellules usagers a été émis et supprimer une cellule vide, sinon émettre une cellule usager en attente ou générer une cellule vide.

To this end, the method comprises the following steps:

• perform a pre-processing on the incoming flow for on the one hand, form a message characterizing each cell received and giving in particular information on the type of cell and on the other hand, select the user cells and store them each for the processing time who will follow,
• perform processing from the messages formed to generate a characteristic cell, send it each time a determined number of user cells has been sent and delete an empty cell, otherwise send a waiting user cell or generate an empty cell.

The treatments carried out in real time according to the method make it possible to carry out cell insertions independently of the flow rate of the incoming flow and without disturbing this flow rate.

• la figure 1, représente le schéma de principe du dispositif conforme à l'invention,
• la figure 2, illustre les mouvements des messages dans la pile 203,
• les figures 3 et 4, illustrent les flux entrants et sortants selon deux modes de réalisation,
• les figures 5, 6 et 7, illustrent des schémas relatifs à plusieurs implantations possibles.

The present invention will be better understood with the aid of the description which follows and which is given by way of illustration and not limitation and with reference to the drawings in which:

• FIG. 1 represents the block diagram of the device according to the invention,
• FIG. 2 illustrates the movements of the messages in the stack 203,
• FIGS. 3 and 4 illustrate the incoming and outgoing flows according to two embodiments,
• Figures 5, 6 and 7 illustrate diagrams relating to several possible locations.

According to the invention, the device comprises a pre-processing circuit 100 for the incident flow of S0 cells.

This circuit makes it possible to form a characteristic ME message in real time for each incoming cell. This message includes in particular an indication of the state or more precisely of the type of cell in order to know whether it is an empty cell or a user cell or another predetermined type of cell. In the following we will qualify all the user cells and non-user cells by non-empty cells. Non-user cells are understood to be cells with a VCI of less than 15 and characteristic cells.

The preprocessing circuit also makes it possible to select non-empty cells from the incoming flow in order to store them progressively.

The device further comprises a processing circuit 200 which, from the stored messages, makes it possible to know when it is necessary to generate a characteristic cell in the output stream. Thus, according to the invention, the circuit makes it possible to generate the characteristic cell and to delete an empty cell from the incoming flow each time that a determined number X of user cells has been sent.

When this number X is not reached, the circuit makes it possible to successively transmit the cells of the incoming stream whether they are user cells or empty cells. In the case where it is necessary to emit an empty cell, the circuit generates this cell since, as we will see below, we do not store the incoming empty cells.

The pre-processing circuit comprises a circuit 101 for detecting user cells and for forming messages for each cell. This circuit further comprises a translation memory 102 and a buffer circuit of the FIFO type. The circuit 101 makes it possible, on the one hand, to detect the user cells from the cell headers to store them in the buffer memory 103 and, on the other hand, to address the translation memory 102.

The cells of the incoming stream are used by the circuit 101 to address the translation memory 102 which each delivers a pre-recorded code. The circuit 101 forms by means of this code and the information of the cell, a message comprising a first word M1 for coding the virtual path number VPI and a second word M2 to indicate the type of cell. A third word M3 contains BIP16 parity bits calculated on the data contained in the cells. The word M2, indicating the type of cell, makes it possible to know whether the cell received is an empty cell or a user cell or a non-user cell, namely for example a maintenance cell or a user cell with a VC <15.

The information S3 circulating between the circuit 101 and the processing circuit 201 of the block 200 correspond to the messages formed by the circuit 101 for each cell received. The processing circuit 200 includes a circuit 201 for generating characteristic cells receiving the information S3 from the message formation circuit. This processing circuit 200 further comprises a cell insertion circuit capable of inserting a characteristic cell in the stream of user cells S5, obtained by reading the buffer memory 103 when it receives the order from the generation circuit 201 .

As an example, we will take the case of the insertion of quality supervision maintenance cells.

We will also take the case where the insertion is done after the X ^th cell of a VPI virtual path. The cell processed by circuit 201 is the cell relating to the message which is located at the bottom of stack 203.

If the value of the message at the bottom of stack 203 corresponds to a cell belonging to a virtual path (among 4096) on which one does not want to carry out a flow insertion or to a non-user cell, then the circuit transmits on S4 to circuit 202 a signal to read the buffer memory 103 and send to S6 this cell and wait for a cell whose message formed by circuit 101 will be placed at the top of stack 203 .

If the value of the message placed at the bottom of the stack corresponds to a user cell of a PV on which we want to insert a characteristic flow, then the circuit increments by 1 the counter 204 of user cells received on this virtual path and performs the treatments necessary for the user cells and the maintenance cell. This processing consists on the one hand, for each cell of each path, to calculate the content of the BIP16 field by making an exclusive OR between the BIP16 of the previous cells and the BIP16 of the current cell and on the other hand, to determine the content of the TUC ("Total User Cell") field. This content is determined by the value of the cell counter.

In the same way as in the previous case, the circuit 201 sends to the circuit 202, a signal of command to read from memory 103, to send to S6 from the user cell and to wait for a cell.

If the value of the message placed at the bottom of the stack corresponds to the X e ^IEM user cell, the circuit 201 transmits S4 on the order to read in the stack 103 the corresponding cell and emit on S6. The processing carried out is the same as that which has just been described above for a user cell (for example, waiting for the next message S3).

In addition, circuit 201 sends an order to generate a maintenance cell to circuit 202. It calculates the BIP16 of this cell and sends it to circuit 202 as well as its VPI, it also sends the MSN content of counter 205 and the total number of user cells TUC given by the counter 204. The content MSN represents the number of the maintenance cell sent. The counter 205 is incremented by 1 for each maintenance cell sent.

The circuit 202 will then send the user cell, then the maintenance cell F4 [VP IO 00 40 20 MSN TUC BIP16 FF FF FF 6A 6A ...... 6A CRCIO], since we have chosen in this example to issue the quality supervision maintenance cell just after the X ^th user cell.

In addition, the circuit 201 increments the counter of emitted maintenance cells 206. It is this counter which will make it possible to know how many empty cells will have to be deleted.

If the value of the message placed at the bottom of the stack 203 corresponds to an empty cell, the circuit 201 sends an order on S4 to generate an empty cell in the case where the counter 206 is zero and waits for the message S3. In the case where this counter is not zero, then the empty cell is not transmitted, the counter 206 is decremented by one and the next cell is immediately processed.

The device which has just been described makes it possible to implement the method according to the invention. According to this method, characteristic cells are inserted into an incident flow of cells.

As has been said, characteristic cells are understood to mean maintenance cells as defined by the ATM standard - for the F4 or F5 flow or specific cells (containing specific data).

The insertion of maintenance flows can be carried out on one or more ATM conduits and even on all the conduits in accordance with the method. In addition, the insertion is carried out independently of the user flow.

According to the method, a characteristic cell is inserted every X user cells of the same virtual path. The number X can be fixed once and for all or variable depending on the chosen application.

This insertion is carried out in accordance with the method while maintaining the flow rate of the user flow. To this end, when X cells of the same path have been sent, we will insert a characteristic cell and delete an empty cell. We will therefore wait for the arrival of an empty cell to delete it after having inserted a characteristic cell.

Of course, if no empty cell is present and the number X of cells is exceeded, the characteristic cell is inserted and the number of empty cells to be deleted is counted. The characteristic cell can be emitted at any time in the interval between cell X and cell (X + 1).

We will now detail the operation of the device. The flow S0 corresponds to the flow of ATM cells. The received cells are detected by their header, the cells having an erroneous header being eliminated. Erroneous headers are detected in a known manner by virtue of the content of the HEC header field. The detection of non-empty cells is conventionally carried out by comparison with the fixed content of an empty cell header.

The flow S1 corresponds to the non-empty cells detected. These cells are put on standby as they arrive in a buffer memory 103. The standby time corresponds in practice to the processing time of the previous cell by the circuit 201. While detecting the user cells, the circuit 101 also makes it possible to form the message ME for each cell of the flow. The first word M1 preferably corresponds to the 12-bit coding of the virtual path indicator VPI and of the virtual channel indicator to which the cell belongs. The second word M2 is the indicator of the type (or state) of the cell.

As has been described, this word M2 makes it possible to indicate whether the cell received is an empty cell, or a user cell, or a non-user cell (typically a maintenance cell). The third word M3 contains 16 bits of BIP16 parity calculated on the data contained in the cell. The message thus formed is transmitted to the processing circuit 201 to be stored in a buffer memory 203.

According to a preferred embodiment, in the case where the cell received is an empty cell or a user cell, the translation memory 102 is used. This memory makes it possible, as has been described, to obtain a word M1 of reduced size compared to the size of the information that would have to be processed since in the absence of this coding we would have 32 bits (VPI + VCI) per cell.

In the case where the cell to be inserted is a maintenance cell as defined for flow F4 in the ATM standard, the word M1 is formed directly without passing through the translation memory, this is the virtual path indicator IPV of the cell.

According to the method according to the invention, these messages are stored in a buffer memory for the processing time of the cells previously placed in the buffer memory. This memory is of the FIFO type. The messages are read successively to be processed by the circuit 201.

Upon initialization, it is expected that the buffer memory 203 comprises a predetermined number M of messages. This introduces a delay of M cells on the real flow, however this delay is small enough not to be constraining.

The messages thus memorized are read one after the other as soon as the number M is reached. When a message ME from a user cell is read, the message from cell X + 1 is stored. The memory in this case always contains M messages.

By reading a message ME relating to a cell X, one knows the content of the word M2 which gives the state of the cell and one can thus carry out the processing which is appropriate.

As long as no characteristic cell is to be inserted, i.e. as long as at least X cells of the same virtual path have not been emitted from the circuit 202, the messages from the buffer memory 203 are continued to be read and the cells read from the buffer 103 are transmitted.

When the message read represents the X ^th user cell, the circuit 202 is instructed to read the FIFO 103, then a characteristic cell is generated and a message S3 is awaited. At the first of the following messages representing an empty cell, this cell is not emitted and the following message is read without waiting for a message corresponding to a new cell to arrive.

The insertion of the characteristic cell was thus compensated for by the suppression of an empty cell; the flow rate of the PV in question is therefore retained.

We will now refer to better understand the diagram in Figure 2.

This figure represents the movements of the messages formed by the cells in the stack 203.

Suppose that the stack is initialized with M = 3 messages. This means that there is no processing during the arrival of these first three messages. Processing begins with the message from cell 1. Suppose it is an empty cell.

Then the circuit 201 transmits to the circuit 202 an indication specifying it to reconstitute the empty cell and to transmit it. This circuit 201 awaits the message from the next cell (cell 14).

This circuit 202 then processes the message from cell 2 which is for example a user cell and waits for the message from the next cell (cell 4).

Suppose that the cell to be treated (cell 3) is the X ^th then, we must send this cell then emit a characteristic cell therefore, during this time, two cells arrive, cells 6 and 7, the messages of which are placed at the top of the stack. At this time, the stack therefore contains M + 1 = 4 messages and the counter of inserted cells is equal to 0 + 1 = 1. The circuit then processes the message from cell 4 which is a user cell and waits for the message from cell 8.

The message of cell 5 indicates that it is an empty cell and the counter of inserted cells is not zero, then this cell is to be deleted, it is not transmitted and the circuit processes the message of the cell 6 next without waiting for the next cell message to arrive. The stack then contains M + 1-1 = M messages and the counter of inserted cells is equal to 1-1 = 0.

Then, the circuit waits for the message from the next cell 9. Two cells were therefore treated during a "cell time".

The message from cell 7 indicates that it is an empty cell and the counter is zero, then this cell is sent and the circuit waits for the next message, ie the message from cell 10 and so right now.

In this example, we have chosen to emit a characteristic cell just after the X ^th cell of the flow. Obviously, this insertion can be deferred by saving the elements necessary for the formation of the characteristic cell and by counting these cells to insert them at the desired time while doing so by deleting an empty cell in accordance with the invention.

Reference may be made for the following to the diagrams in FIGS. 3 and 4.

FIG. 3 illustrates, diagrammatically on the first line, an incident flow in which the cells from 1 to 13 have been numbered and in which the different types of cells have been shown separately according to the legend indicated. The second line represents the resulting flow, after insertion of a maintenance cell and deletion of an empty cell at the indicated location, ie after the X ^th cell of the same VP56 virtual path (for example).

Figure 4 illustrates a second alternative embodiment. According to this variant, a maintenance cell is introduced into the incident flow shown on the first line so that it is in the middle of two cells of the same VP56 conduit, the first being the X ^th cell of this VP56.

The medium between two cells of the same conduit is defined by the distance which separates them in time, ie the flow. This distance is easily measurable since it involves measuring the flow rate and dividing by two to find out the location of the cell to be inserted.

The diagrams of FIGS. 5, 6 and 7 illustrate three possible implantations of the device which has just been described.

Figures 5 and 6 correspond to cases where the insertion of cells is done in ATM equipment.

Indeed, it is possible to install this device in a brewer or in a switch 400 by placing this device upstream of the brewer or of the switch as shown in FIG. 5.

The interface 300 represents the adaptation between the ATM level and the physical level.

In addition, this device can also be installed on a network terminal, and in this case it will be placed at the output of the terminal as shown in FIG. 6.

FIG. 7 corresponds to the case where the insertion is made outside of ATM equipment.

In this case, the device is placed on the network line. It receives the flow transported by this line and emits the flow on the network line after insertion of the cells (for maintenance). In this case, a measuring device 500 is provided on reception to carry out network quality measurements.

• pour les circuits 101 et 202 : on a utilisé des circuits à logique programmable XILINX,
• pour le circuit 201 : on a utilisé un processeur rapide du type DSP qui permet d'effectuer des traitements en temps réel (temps cellule). Avec des processeurs rapides du type DSP, on peut insérer un flux F4 ou F5 ainsi qu'un flux spécifique à n'importe quel débit.

As an example, the device was produced in the following manner:

• for circuits 101 and 202: circuits with XILINX programmable logic were used,
• for circuit 201: a fast processor of the DSP type was used which makes it possible to carry out processing in real time (cell time). With fast processors of the DSP type, one can insert an F4 or F5 stream as well as a specific stream at any bit rate.

Claims (8)

Method for inserting a flow of characteristic cells into an incident cell flow conforming to the ATM standard or the like whatever its speed, the flow comprising in particular user cells and empty cells, characterized in that it comprises the steps following: - perform pre-processing in real time on the incoming flow, on the one hand, form a message characterizing each cell received and giving in particular information on the cell type and, on the other hand, select the user cells and store them each the processing time that will follow, - carry out a processing in real time from the messages formed to generate a characteristic cell, send it each time a determined number of user cells has been emitted and count the characteristic cells emitted to delete a corresponding number of empty cells, otherwise send a waiting user cell or generate an empty cell. Method for inserting a flow of cells according to claim 1, characterized in that the insertion of a characteristic cell on a virtual path is made immediately after the appearance of the X ^th cell of the flow and, the deletion of the empty cell takes place as soon as an empty cell appears on the flow after this insertion or during. Method for inserting a cell stream according to claim 1, characterized in that the insertion of a characteristic cell on a virtual path is made at any time in the time interval which separates the X ^th cell from the path and the (X + 1) ^th , and in that in the absence of an empty cell at delete, their number is counted. Method for inserting a flow of cells according to any one of the preceding claims, characterized in that the cells inserted are quality supervision maintenance cells (F4 or F5). Method for inserting a flow of cells according to any one of the preceding claims, characterized in that the cells inserted are cells of a specific flow defined for a particular application. Device for inserting a flow of cells according to any one of the preceding claims, characterized in that it comprises: - real-time pre-processing means making it possible to form a message for each incoming cell, these means also making it possible to detect the user cells for storing them, - real-time processing means making it possible to count the user cells, to generate the characteristic cells, to emit a characteristic cell after the emission of X user cells and to delete an empty cell. Device for inserting a flow of cells according to claim 6, characterized in that the pre-processing means comprise means for detecting user cells, a translation memory and a buffer memory. Device for inserting a cell stream according to claim 6, characterized in that the processing means comprise means for generating characteristic cells, a buffer memory and means for inserting the cells generated in the output stream.

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